Pre-heat the oven to 350°.
If you're making buttermilk, make it first and set aside.**
Cream (or mix) the butter, oil, and sugar together for a good 5 to 8 minutes.
You can also cream the egg yolks and vanilla extract in with the sugar mixture if you want, or you can add them in after the butter and sugar mixture is creamed.
Meanwhile, sift the dry ingredients together at least 3 times.
Then alternate adding the dry ingredients with the buttermilk to your creamed mixture, mixing after each addition.
Finally, when everything is mixed together, fold in the egg whites and mix together just until the batter is blended.
Add cupcake liners to 2 large muffin tins, and pour or dispense the batter into each cup, filling each cup 3/4 of the way full.
Bake at 350° for about 20 minutes, or until a toothpick or cake tester comes out clean.
Let the cupcakes cool in the pan for about 10 minutes, then remove to a wire rack.***
Allow the cupcakes to fully cool before attempting to ice them. Then top with this deliciously creamy butterbeer buttercream frosting.****